1. Field of the Invention
The present invention relates to a mobile communication system and in particular, relates to a mobile communication system adopting CDMA.
2. Description of the Related Art
Recently, mobile communications have widely spread. In a mobile communication system, each mobile station is accommodated in the nearest base station and is connected to a network through the base station. In this case, since each base station accommodates as many subscribers as possible, usually radio signals are multiplexed when transmitted. For the multiplex method of radio signals, conventionally, Frequency-Division Multiple Access (FDMA) or Time-Division Multiple Access (TDMA) is adopted. However, in the next generation system, Code-Division Multiple Access (CDMA) is anticipated to become mainstream.
In CDMA, each channel is identified by a code. Specifically, when transmitting a signal, a transmitter (mobile station or base station) spreads the spectrum of the signal by multiplying the signal by a specific code. Then, a receiver (base station or mobile station) decodes the signal by multiplying an incoming signal by the same code as that used by the transmitter. Therefore, in CDMA, a specific frequency band is shared by a plurality of channels.
In a mobile communication system, generally, hand-off (or hand-over) is performed. Hand-off means to change the base station accommodating a mobile station from a source base station to a target base station when the mobile station accommodated in a specific base station (the source base station) travels to the communications area of another base station (the target base station). In this case, whether hand-off should be performed is decided, for example, based on the power of an incoming radio signal at the mobile station. In this case, if the power of a signal from the source base station at the mobile station drops below a specific threshold value and simultaneously if the power of a signal from the target base station exceeds a specific threshold value, hand-off is performed.
Although currently a mobile communication system is mainly used for voice communications, it is predicted that the amount of data communications will become high in the future. Voice communications are conducted at low and most often constant speed, while data communications are conducted at high and variable speed. Therefore, if a hand-off method presuming voice communications is adopted in a mobile communication system mainly used for data communication, there is a possibility that a variety of inconveniences may occur. In addition, if a communication resource management method presuming voice communications is adopted in a mobile communication system mainly used for data communication, there is a possibility that communication resources will not be efficiently used.
However, neither a detailed hand-off method for data communications (packet data communications and the like) nor a detailed communication resource management method for data communications have been proposed. In particular, such methods have not been proposed in a mobile communication system adopting CDMA.